In many wireless communication systems, RF signals are down-converted to an IF signal and/or baseband signal prior to conversion to digital signals. Filters are conventionally used to remove interferers from the IF or baseband signal in order to decrease the signal's dynamic range which may be helpful in the subsequent conversion from an analog baseband signal to a digital signal. With the move from super-heterodyne receivers to direct-down conversion receivers, the filtering has moved from being off-chip passive filtering at the IF frequency to being on-chip active filtering at a baseband frequency. One important parameter in receivers is noise figure. Because the noise figure of baseband filters may, in some cases, significantly affect the overall receiver noise figure, a baseband filter's noise figure should be kept as low as possible. Unfortunately, this conventionally has required that on-chip capacitors used in the baseband filters be large in value to limit the added noise. Large on-chip capacitors, however, typically require a significant amount of the area of an RF integrated circuit.
Some conventional approaches have used the gate capacitance of MOSFETs to achieve higher-density and thus smaller capacitors (e.g., MOSCAPs); however these conventional approaches typically have to change the semiconductor manufacturing process to build a permanent channel under the gate so that a capacitance can be provided with no bias voltage. The additional masking step in the fabrication process for this channel can significantly increase the cost of an RF integrated circuit (RFIC).
Some other conventional approaches use MOSCAPs coupled to ground with a high-common-mode signal. These approaches generally require doubling of the capacitance for use in a differential circuit, which offsets at least some of the size reduction obtained from the density increase.